1. Field of the Invention
This invention relates to the structure of a capacitor employed in a semiconductor integrated circuit, and more particularly to a capacitor formed by using a ferroelectric substance, and a non-volatile memory using the capacitor.
2. Description of the Related Art
As for a non-volatile memory of this type, a memory cell has been disclosed, for instance, by Unexamined Japanese Patent Application Hei-2-304796/(1991) FIG. 5 shows an electrical equivalent circuit of the memory cell, and FIG. 6 shows the structure of the same.
The memory cell, as shown in FIG. 5, comprises: a switching element, namely, a field-effect transistor 10, and a signal charge storing capacitor 20 using a ferroelectric substance. The field-effect transistor 10 has a gate electrode 11, a drain electrode 12, and a source electrode 13. The gate electrode 11 is connected to a word line WL, and the drain electrode 12 is connected to a bit line BL. The capacitor 20 comprises: a ferroelectric film 23; and two electrodes 21 and 22 formed on both sides of the ferroelectric film 23, respectively. The electrode 21 is connected to the source electrode 13 of the field-effect transistor 10, and the electrode 22 is connected to a ground line Vss or to a drive line DL. The ferroelectric film 23 is, in general, made of lead zirconate-titanate (called "PZT").
The structure of the memory cell thus organized will be described with reference to FIG. 6 in brief.
A field oxide film 2 is formed by selective oxidation of the surface of a silicon substrate 1, thus defining a element forming region. In the region, the field-effect transistor 10 is formed which consists of a gate electrode 11 covered by an oxide film 3, a drain region 12a, and a source region 13a. The lower electrode 21, the ferroelectric film 23, and the upper electrode 22 are formed on the source region 12a in the stated order, to form the capacitor 20. A metal conductor 4 is formed, as the bit line BL, on the drain region 12a, and a metal conductor 5 is formed, as the ground line Vss or the drive line DL, on the upper electrode 22.
The storage of charge of the ferroelectric capacitor in the above-described conventional non-volatile memory will be described with reference to FIGS. 7 and 8. FIG. 7 is an explanatory diagram showing the conventional capacitor formed on a semiconductor substrate. In FIG. 7, reference characters a and b designate the terminals of the capacitor. When voltage is applied across those terminals a and b of the capacitor, an amount of charge stored in the ferroelectric film 23 between the electrodes 21 and 22 is as shown in FIG. 8, in which the horizontal axis represents field strengths E and the vertical axis, amounts of polarization P. As the voltage between the terminals a and b changes, the amount of polarization of the ferroelectric film 23 changes as O.fwdarw.A.fwdarw.B.fwdarw.C.fwdarw.D.fwdarw.E.fwdarw.F.fwdarw.G.fwdarw.B, thus showing a hysteresis characteristic.
When the field strength between the electrodes 21 and 22, after being raised to E.sub.sat much larger than E.sub.O, is returned to O, then an amount of polarization P.sub.s (called "spontaneous polarization") remains in the ferroelectric film 23. Similarly, when the field strength between the electrodes 21 and 22, after being decreased to -E.sub.sat, is returned to O, then an amount of polarization -P.sub.s remains in the ferroelectric film 23. With those positive and negative spontaneous polarizations corresponding to data "1" and "0" write states, the capacitor 20 provides a read signal charge Q represented by the following equation:
Q=2 P.sub.s .multidot.S (Coulomb)
where S is the capacitor's area.
The spontaneous polarization P.sub.s is determined from characterization such as composition and thickness of the ferroelectric film 23.
The conventional non-volatile memory designed as described above suffers from the following difficulties:
As shown in FIG. 9, in general, the switching time of a capacitor formed with a ferroelectric substance such as PZT is decreased as the electrode area decreases. This is a merit provided when the electrode area is decreased for integration. However, as shown in FIG. 10, as the electrode area decreases, the inversion charge density (corresponding to 2 P.sub.s in the above-described equation) is decreased abruptly, as a result of which it is difficult to read the signal charge Q.